Atmospheric air is recognized to comprise a number of different gaseous components, primarily nitrogen and oxygen, but also minor amounts of other materials, such as noble gases, methane, water vapor, and carbon dioxide. One or more of the components of atmospheric air can be separated and provided in a purified form through use of an air separation method and system—i.e., an “air separation unit” or “air separation plant”. There are various known technologies that are used for the separation process, such as cryogenic distillation of purified air, oxygen ion transport membrane (or other membrane) separation, pressure swing adsorption (PSA), and vacuum pressure swing adsorption (VPSA). Of the various available methods, cryogenic distillation can be particularly advantageous for the separation of air into its constituent parts at high purity. Known cryogenic air separation units achieve the low distillation temperatures required through use of a refrigeration cycle with the cold equipment maintained within an insulated enclosure. The cooling process and the separation of the air components typically requires a large amount of shaft power to drive the air compressors used in the refrigeration and separation cycles. Air separation units also can require initial separation of water vapor, CO2, and other minor components to avoid freezing thereof within the cryogenic equipment.
A typical cryogenic air separation process can include four main features: air compression; air purification; heat interchange between cooling air streams and heating products of air separation; and distillation. In the compression stage, the total feed of atmospheric air is pre-filtered and compressed to a pressure typically from 3.5 to 10 bar (0.35 to 1 MPa). This compression stage imparts heat to the air, and such added heat must be removed (e.g., in a heat exchanger) to lower the compressed air temperature to around ambient temperature. The air is generally compressed in a multi-stage compressor with inter-cooling between stages and an after-cooler using an ambient cooling means, such as cooling water or air. The heat of compression is rejected to the environment. Purification of the compressed air can be achieved by passage through an adsorption process, such as cyclically operating beds of alumina and/or molecular sieve adsorbents. This can be useful to remove any remaining water vapor, CO2, and any other components that would be subject to freezing in the cryogenic equipment, as well as gaseous hydrocarbons. The adsorbents can be regenerated by methods, such as thermal swing or pressure swing using dry nitrogen purge gas at low pressure. Cooling and distillation begins with passage of the air streams through an integrated heat exchanger (e.g., an aluminum plate fin heat exchanger) and cooling against produced oxygen and waste cryogenic product streams. The air is then cool enough to be distilled in a distillation column. The formation of liquid air in the cryogenic equipment typically requires some refrigeration. Such liquid may be formed by Joule Thomson expansion of air across a valve or through an expander. The air is distilled in at least one and often two or three distillation columns, depending on the requirements for products to be provided, product purities, and product delivery pressures. The separated air product components can be warmed against the incoming air in the heat exchangers to provide the product gases at ambient temperature.
The production of oxygen at elevated pressure is accomplished in all current cryogenic air separation systems using the pumped oxygen process. This involves pumping a stream of low pressure liquid oxygen taken from the distillation system to the high pressure required by a downstream oxygen consuming process. The high pressure liquid oxygen is vaporized and heated to ambient temperature by heat exchange with a portion of the air feed stream that has been compressed to a pressure sufficiently high to give a low temperature difference between the oxygen and air streams in the heat exchanger. Generally between about 27% and 40% of the purified total air feed stream is compressed in a second multi-stage air compressor to a pressure which depends on the required oxygen pressure. The air pressure can be about 27 bar (2.7 MPa) for oxygen at a pressure of 10 bar (1.0 MPa), and the air pressure can be about 100 bar (10 MPa) for oxygen at a pressure of about 300 bar (30 MPa).
Air separation units can be stand-alone systems providing bulk products for commerce. Alternately, air separation units can be integrated with other methods and systems where a continuous stream of products separated from the air is required. Specifically, air separation units can be integrated with combustion systems wherein fuel is combusted for power production and a purified O2 stream is required to facilitate combustion. Because of the ever increasing need for power production in a growing world economy, there remains a need in the art for improved air separation methods and systems, particularly methods and systems that can be usefully integrated into power production methods and systems.